Stirling engine assembly

ABSTRACT

A Stirling engine assembly comprising a Stirling engine ( 1 ) with a hot head ( 3 ) and a cold region ( 4 ). An annular burner ( 9 ) surrounds the head and is arranged to provide heat to the head. A corrugated seal ( 2 ) between the Stirling engine and the burner prevents the flow of combustion gases from the head into the surrounding environment. The Stirling engine is supported by a mounting frame ( 23 ) at least in part via the seal.

The present invention relates to a Stirling engine assembly. Inparticular, the invention relates to an assembly suitable for use in acombined heat and power (chp) unit.

Where an appliance, such as a chp unit, is installed in a domesticenvironment, it is vital that noise and vibration that could cause aconsiderable nuisance, is kept to a very low level. As the appliancecontains a Stirling engine, combined with an alternator, it produces aconsiderably higher level of noise and vibration than would beacceptable. It is therefore necessary to minimise the transmission ofnoise and vibration to the domestic environment, through the casing andsupport frame of the combined heat and power unit.

A Stirling engine burner is located around the heater head at the top ofthe engine. A problem for the Stirling engine-based chp system is theneed to ensure that combustion gases do not flow downwards into theroom-sealed unit enclosure, causing the accumulation of potentiallyharmful gases. Some form of seal is therefore required between theStirling engine and the burner casing.

When operating, the Stirling engine vibrates, due to its reciprocatingcomponents. A vibration reduction system, incorporating various dampingand absorbing components can bring the residual levels of vibration to alow level, but there is still enough to cause problems to any seallocated between the vibrating engine and the stationary burner casing.The seal design is required to be extremely robust, operate at hightemperatures, and be capable of maintaining an adequate seal under alloperating conditions, as defined by the gas appliance certificationprocedure. Some conventional seal designs are typically significantlystiffer than the engine suspension system and would, if used in theapplication, lead to unacceptable transmission of forces between theoscillating engine and the static burner components.

Excessive wear, fatigue or degradation of such a seal would causecombustion gases to leak into the unit enclosure, causing a hazard, andincreasing noise levels.

U.S. Pat. No. 5,918,463 discloses a Stirling engine with a washer shapedpiece of flexible, semi rigid, or rigid fibrous ceramic insulationbetween the burner casing and Stirling engine.

The usual practice is to support an engine by mounting it on top ofsprings, which isolate a large proportion of the vibration producedduring normal engine operation. An example of a Stirling engine havingsuch an arrangement is U.S. Pat. No. 4,400,941. To maximise the degreeof isolation, a low stiffness mounting system is required. Theimplementation of this, with compression springs, can lead toinstability, especially where the forces involved are lateral in naturein addition to vertical oscillations. An alternative support arrangementis therefore necessary. Our previous patent application PCT/GB 02/05111details a solution to this problem, where springs are arranged aroundthe outer surface of the Stirling engine, to suspend the engine from amounting flange.

According to the present invention, there is provided a Stirling engineassembly comprising a Stirling engine with a hot head and a cold region,an annular burner surrounding the head and arranged to provide heat tothe head, and a corrugated seal between the Stirling engine and theburner to prevent the flow of combustion gases from the head into thesurrounding environment, wherein the Stirling engine is supported by amounting frame at least in part via the seal.

The seal design can thus be made to be flexible enough to cope with therelative motion (both vertical, horizontal and rotational in nature)between engine and burner. In addition, suitable materials for the sealare available which can withstand the high temperatures associated withthe burner gases, and are not corroded by the gases involved.

By supporting the Stirling engine by a mounting frame at least in partvia the seal, an arrangement is provided which supports the engine whichisolates a large proportion of the vibration, while, at the same time,providing a highly effective seal preventing combustion gases fromescaping into the body of the chp unit casing.

As part of the weight of the Stirling engine is supported by the seal,the suspension system can be made lighter as it supports less weight, orcan even be removed altogether with obvious cost advantages.

Insulation is preferably provided between the seal and the engine tosubstantially reduce the passage of hot combustion gases from the burnertowards the bellows.

The seal may, for example, be a bellows.

The bellows may be arranged such that it extends from a locationadjacent to the burner, along a substantial portion of the length of theStirling engine. In this case, means are provided for passing coolantthrough the bellows to provide a flow of coolant liquid to and from anengine cooler. This preferably entails a coolant inlet and coolantoutlet pipe extending through the bellows and being sealed by a flexibleseal. The bellows is preferably provided in this region, with acylindrical portion. This elongate bellows design reduces the levels oftransmitted noise from the Stirling engine by providing a sealed gascushion around the body of the engine. In the same way, however, thisgas cushion may insulate the engine and reduce heat losses from thecasing. As the alternator, in particular, relies on air cooling aroundthe lower engine/absorber casing to maintain the temperature of themagnet at an operational level, this may be disadvantageous. To overcomethis, it is possible to add cooling fins to the exposed lower end of theengine to aid heat loss, thereby compensating for the warming effect ofthe bellows.

As an alternative to the bellows extending along a substantial length ofthe engine, the bellows may terminate above an engine cooler. In thiscase, there is no need for the coolant to pass through the bellows.

If the bellows is arranged to extend vertically, the weight of theStirling engine is borne along the length of the bellows. However, thebellows may be arranged at an angle to the vertical.

The weight of the Stirling engine may be borne entirely by the bellows.Alternatively, the weight of the Stirling engine is borne partially bythe bellows and partially by one or more additional resilient members.Such as springs from which the engine is suspended.

Examples of Stirling engine assemblies in accordance with the presentinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic section of a first example;

FIG. 1 a shows a portion of FIG. 1 in greater detail;

FIG. 2 is a view similar to FIG. 1 showing a second example;

FIGS. 3 a and 3 b are cross-sections of alternative bellows sections;

FIG. 4 is a cross-section through part of a Stirling engine showing analternative seal configuration which is not in accordance with thepresent invention;

FIG. 5 is a view similar to FIG. 4 showing a further bellowsarrangement;

FIG. 6 is a view similar to FIG. 2 showing a third example;

FIG. 7 is a view similar to FIG. 2 showing an example which is not inaccordance with the invention; and

FIG. 8 is a section through line VIII-VIII in FIG. 7.

The Stirling engine assembly comprises a Stirling engine 1 housed withina casing 2. The design of the Stirling engine 1 is well-known in theart. The engine is broadly divided into three segments, a heater head 3,a cooler 4 and an alternator 5. The engine has displacer and powerpistons, both of which are arranged to reciprocate in a verticaldirection. This produces a net vertical vibration of the Stirling engineitself. In order to reduce this vibration, an annular absorber mass 6 issupported by a number of compression springs 7 both above and below theabsorber mass.

In order to transfer heat to the heater head 3, a gas/air mixture issupplied along an inlet duct 8 to a burner element 9 where it isignited. The heat generated is transferred to a heater head 3 via aplurality of annular fins 10. The combustion gases flow up through thefins 10 around the top of the heater head and into a recuperator 11 inwhich they preheat the incoming gas/air mixture and subsequently heatwater for domestic use. Ceramic fibre insulation 12 increases theresistance to downward gas flow so that very little downward gas flowoccurs.

The combustion gases are prevented from escaping into the externalenvironment by the presence of an annular seal in the form of bellows 20surrounding the Stirling engine 1. At its top end, the bellows has anannular flange 21 which is bolted to the lower surface of theburner/recuperator assembly 22. This flange 21 sits on the unit frame23. This frame 23 is a rigid box frame attached to the wall of adwelling. At its lower end, the bellows 20 terminates in a lower annularflange 24 which is bolted or connected using a clamping ring to amounting ring 25 which is welded around a lower portion of the casing 2of the Stirling engine 1 adjacent to the alternator 5. In this way, theweight of the Stirling engine 1 including the fins 10 together with thevibration absorber 6 and its associated mountings are all supported onthe unit frame 23 via the bellows 20.

In order to circulate cooling liquid around the cooler 4, it isnecessary to provide flow of coolant to and from the cooler. An annularcoolant duct 30 surrounds the casing 2 in the vicinity of the cooler 4.This annular duct is fed with coolant liquid from an inlet pipe 31,while the outlet from the duct 30 is via outlet pipe 32. The inlet 31and outlet 32 pipes extend through the wall of the bellows 20 as shownin greater detail in FIG. 1 a. At this location, the wall of the bellows20 is cylindrical and is provided with a pair of circular openings 33. Arigid pipe extension 34 which is screwed to the annular duct 30 passesthrough the opening 33. The inlet 31/outlet 32 pipe (as the case may be)is fastened to the rigid pipe extension 34 using a jubilee clip with aclamping ring 35. At each opening 33, a seal is made using a flexiblerubber grommet seal 36. This seal presses against the rigid pipeextension 34. This arrangement will allow the pipes 31/32 to vibratewithout damage. As the grommet seals 36 are in contact with the coolantpipes, the temperature in this region is low enough to allow the use ofa commercially available rubber seal, giving low rates of wear forcomponents in this area.

As it is not intended that the Stirling engine should be serviced onsite, there is no requirement for access to the components that will besealed within the bellows 20. If an engine failure occurs, the enginewill be removed, repaired and replaced as a single module (includingbellows). The rigid pipe extensions 34 and grommet seals 33 could,however, be replaced at service intervals.

A second example of a Stirling engine assembly in accordance with thepresent invention is shown in FIG. 2. This is largely the same as theexample shown in FIG. 1, with the same reference numerals having beenused to designate the same components. A further description of thesecommon components is not repeated here.

The second example differs from the first example in that the bellows20′ terminates above the cooler 4. In this case, the upper mounting isthe same as for the first example, but the lower mounting is via amounting plate 24′ welded around the casing 2 above the cooler 4. Inthis case, neither the annular absorber mass 6, nor the annular coolantduct 30 are within the bellows. There is therefore no need to provide aninterface between the coolant inlet 31/outlet 32 pipes and the bellows.

With this arrangement, the Stirling engine 1 including the fins 10together with the vibration absorber 6 are suspended from the unit frame23 via the bellows 20′.

The bellows 20 consists of a flexible stainless steel (AISI 32 or AISI316Ti) tube with annular corrugated convolutions. The most costeffective cross-sectional shape of bellows is the rounded-end section ofFIGS. 1 and 2 and as shown in more detail in FIG. 3 a. These are made bya hydraulic forming process. Alternatively, the cross-section may havesharp edges 40 which are each welded. The rounded section bellows alsohas more advantageous properties in terms of allowing relative lateralmovement between its ends. This can be important where vibrationalforces produced by the Stirling engine can be horizontal as well asvertical and reduces transmission of forces within the system.

Typically, the weight of the Stirling engine 1 and absorber mass 6 is 20to 100 kilograms. The stiffness of the bellows is adjusted to match theengine weight and also the space available for allowable extension.

Typically, for the engine of FIG. 1 there will be 3 to 4 convolutionsabove the cooler 4 and 12 to 18 convolutions beneath the cooler 4. Inthe short bellows of FIG. 2, there will be typically 3 to 4convolutions. The stiffness per convolution is 380N/mm to 50N/mm for a60 kilogram engine. The stiffness per convolution is varied by alteringthe outside diameter of the bellows, while keeping the inside diameterconstant. Lower stiffness has the advantage of reducing vibrationlevels, but needs to be balanced against the additional weight and theextra space needed around the engine.

Alternatives to the vertically extending bellows are shown in FIGS. 4and 5. These examples are, in all other ways, similar to FIG. 2.

FIG. 4 shows an annular disc 20″ with concentric annular convolutionswhich are convoluted in a direction perpendicular to the plane of thedisc. In this case, mounting plate 24″ has an upwardly extending annularflange 50, while a downwardly annular flange 51 depends from the casingof the burner 9. The seal 20″ is mounted between these two flanges andheld in place with annular clips 52. Such an arrangement, however, isnot capable of supporting any of the weight of the engine and istherefore not a part of the present invention.

A similar arrangement is shown in FIG. 5, but in this case the seal is abellows 20′″ which is angled at around 45° to the horizontal.

In the examples described to date, all of the weight of the Stirlingengine 1 and absorber mass 6 is suspended through the seal 20. As analternative, as shown in FIG. 6 the seal may bear some of the weight ofthe Stirling engine 1 and absorber mass 6, while some additionalsuspension for the Stirling engine 1 and absorber mass 6 is provided.This may be in the form of a plurality of springs 60 which are arrangedaround the engine and are attached between the unit frame 23 and thelower flange 24. This allows the size and therefore the weight and costof the bellows to be reduced. In this case, spring failure would not beas serious as failure of the bellows suspension, so that thisarrangement reduces risk of costly chp down-time. However, the weightand cost reduction of the seal must be balanced against the additionalcomponents required with this arrangement with their associatedadditional weight and cost. Although FIG. 6 is shown with a bellowssimilar to that in FIG. 2, it would also be possible to use any of thealternative configurations of FIGS. 1 and 3 to 5.

FIG. 7 shows a Stirling engine assembly in which the Stirling engine ismounted horizontally. Most aspects of the Stirling engine assembly aresimilar to that shown in FIG. 2 and are not described in further detailhere. In this case, the bellows 20 is acting purely as a seal and doesnot bear any weight of the Stirling engine assembly. This exampletherefore does not form part of the present invention. The annular disc20″ of FIG. 4 is particularly suited to this type of horizontalmounting.

Instead, the weight of the assembly is carried by a support 70. Thiscomprises two arcuate brackets 71, 72 attached to the engine 1 adjacentto the cooler 4 and to the end of the alternator 5 respectively. Coolingpassages 73 within the brackets 71, 72 permit the flow of air andprevent the temperature of the casing 2 adjacent to the alternator fromrising to unacceptable levels. Legs 74 extend from each of arcuatebracket into a base 75 in which they are retained in by rubber seats 76to reduce the transmission of vibration to the base 75.

1. A Stirling engine assembly comprising a Stirling engine with a hothead and a cold region, an annular burner surrounding the head andarranged to provide heat to the head, and a corrugated seal between theStirling engine and the burner to prevent the flow of combustion gasesfrom the head into the surrounding environment, wherein the Stirlingengine is supported by a mounting frame at least in part via the seal.2. An assembly according to claim 1, wherein insulation is providedbetween the bellows and the engine to substantially reduce the passageof hot combustion gases from the burner towards the seal.
 3. A Stirlingengine assembly according to claim 1, wherein the seal is a bellows. 4.An assembly according to claim 3, wherein the bellows extends from alocation adjacent to the burner, along a substantial portion of thelength of the Stirling engine.
 5. An assembly according to claim 4,wherein means are provided for passing coolant through the bellows toprovide a flow of coolant to and from an engine cooler.
 6. An assemblyaccording to claim 5, further comprising a coolant inlet and a coolantoutlet pipe extending through the bellows and being sealed by a flexibleseal.
 7. An assembly according to claim 4, wherein the bellows iscylindrical in the region of the engine cooler.
 8. An assembly accordingto claim 3, wherein the bellows terminate above the engine cooler.
 9. Anassembly according to claim 3, wherein the bellows is arranged to extendvertically such that the weight of the Stirling engine is borne alongthe length of the bellows.
 10. An assembly according to claim 3, whereinthe bellows is arranged at an angle to the vertical.
 11. An assemblyaccording to claim 3, wherein the weight of the engine is borne entirelyby the bellows.
 12. An assembly according to claim 3, wherein the weightof the Stirling engine is borne partially by the bellows and partiallyby one or more additional resilient members.
 13. An assembly accordingto claim 12, wherein the additional resilient members are springs fromwhich the engine is suspended.